using a cad designed integrated sub-mm wave ...by using a superconducting integrated sub-mm wave...

Fourth International Symposium on Space Terahertz Technology Page 485

LINEWIDTH MEASUREMENTS OF FLUX-FLOW JOSEPHSON OSCILLATORS

USING A CAD DESIGNED INTEGRATED SUB-MM WAVE RECEIVER

Y.M. Zhang, D. Winkler, and T. ClaesonDepartment of Physics

Chalmers University of TechnologyS-412 96 Goteborg, SWEDEN

ABSTRACT

By using a superconducting integrated sub-mm wave receiver, the linewidth of a flux-flow typeJosephson oscillator (FFO) was measured to be less than 2.1 MHz in the band 280-330 GHz.The output power coupled to a 10 i2 microstripline was about 0.5 jiW, and could be adjusted bychanging the current bias of the oscillator junction.

The integrated receiver consists of two identical Josephson flux-flow oscillators and an SIS(superconductor-insulator-superconductor) mixer. We can determine the composite linewidth ofthe oscillators by mixing them down to an intermediate frequency in the 515 mixer. The FPOsare connected to the SIS mixer via centerfed interdigital capacitors. One FFO is well coupled tothe SIS mixer by using a 3-step Chebychev rnicrostripline transformer and a fairly largeinterdigital capacitor, while the second H40 has about 15 20 dB weaker coupling. A stripinductor, rf-terminated by a radial stub resonates out the SIS junction capacitance at about340 GHz. Radial stubs were also used as RF chokes for the if-line (4 GHz). The FFOs and theSIS element are made of Nb/Nb-oxide/PbBi tunnel junctions.

The circuit was optimized by careful CAD modeling (HP Microwave Design System). Normalconductors were used with the resistivity set to zero. Microstripline widths and lengths wererecalculated for the superconducting case with the London penetration depths of the electrodes(Nb and PbBi) taken into account. After adjusting the area of the SIS element and the length ofthe tuning strip inductor, a good response was observed from 280 to 330 GHz, with the LCresonance at 340 GHz. The power was high enough to pump the SIS mixer.

1. INTRODUCTION

For space born missions, low weight and small volume are required. When it comes to imagingarrays, this is even more of a concern. By using SIS (superconductor-insulator-superconductor)mixers with much less local oscillator (LO) power requirements some of the problems can bealleviated. Even better would be if the external LO could be replaced with an on-chip fullyintegrated source. The concept of using superconducting technology for the construction oflow-noise, integrated millimeter or submillimeter-wave receivers has been considered as apromising idea for many years. However, not until recently has sufficient progress been made to

7

Page 486 Fourth International Symposium on Space Terahertz Technology

really demonstrate the possibilities. One way to achieve practical oscillators is by using coherent,mutually phase-locking arrays. Bi et al. [1] measured a 10 MHz linewidth by mixing twoJosephson array oscillators at 250 GHz. Sufficient available power existed to pump an SISmixer, but the number of junctions in the oscillator or the coupling strength to the SIS mixer hadto be fixed at the design stage to obtain the optimum local oscillator power for the designfrequency.

Another way to achieve a practical oscillator is by using the fluxon (flux quantum (130=h/2e)propagation in a long Josephson tunnel junction (LB), either in a resonant mode (back and forthpropagation of solitons) or in a unidirectional flux-flow mode. A soliton (fluxon) is anelectromagnetic pulse and gives rise to the emission of a pulse of electromagnetic energy when ithits the end of the junction. If the arrival of solitons at one end of the junction is periodic, rfradiation is emitted. The propagation mode of the fluxon in a LJJ depends on the junction qualityfactor Qj, which is related to the junction losses as [2]

QJ = ((coo/0).0 2 + G)' (1)where coo= (27c/00 )Vdc is the frequency of Josephson oscillation (Vdc is the dc voltage of thejunction, (oj is the Josephson plasma frequency, 13 is the surface loss, and a is the quasiparticleloss).

The first type offluxon oscillator is called resonant soliton oscillator (RSO), which uses a longjunction with a high quality factor Qj (low quasiparticle loss a). In order to achieve a high Qj,the junction should have a low critical current density jc [3]. Driven by the bias current, a fluxontravels along the junction until it reaches the junction boundary, where it is reflected as an anti-fluxon, which in turn is driven in the opposite direction. This process is repeated, resulting in amicrowave emission at a frequency given by [4]

f = Vdc 200 =nul2L (2)

where u is the average fluxon velocity, n is the number of fluxons moving in the junction oflength L. A RSO is operated at zero or a relatively weak external magnetic field. Its oscillationfrequency is determined mainly by the junction length L rather than the applied field and bias

F i g.1 Computer simulation showing time-dependent junction voltage V(x,t) of a LJJ in the single resonantsoliton (n=1) case for parameters: Lai =12, 0:=0.05, 13 =0 . 0 2 , y=0.35 (at zero magnetic field).


current. This suggests a rather well defined frequency. The resulting resonant step in thecharacteristic is commonly called a zero field step (ZFS). When the oscillator is biased on a zerofield step, the emitted radiation is characterized by a very narrow output linewidth - about 1 kHzat 10 GHz [5]. A main weakness of this kind of oscillators is its low output power - reportedpowers coupled to SIS detectors are less than lOnW at —100GHz [6]. However, Cirillo et al.[7]have estimated a power of —100 nW at 75 GHz into a tightly coupled small junction. The lowoutput power is partially due to the low jc requirement for a RSO. Also, the output voltagewaveform of a RSO consists of fairly sharp spikes, which suggests that the output power isdelivered into a broad range of harmonics rather than being concentrated at the fundamentalfrequency. In Fig.1, we show a computer simulation for the single soliton propagation in a 1_,JJwith overlap geometry. Note that this figure is in normalized units: distance x is normalized tothe Josephson penetration depth k j , time is normalized to the inverse of wj, and 7 is the biascurrent 1B normalized to the maximum zero-voltage Josephson current

I. Another weakness of aRSO is its upper oscillation frequency limited by the junction length L which should be muchlarger than the Josephson penetration depth Ad . This makes it difficult to operate at sub-mmwavelengths.

The second type offluxon oscillator is called flux-flow Josephson oscillator (FFO). It is basedon a long Josephson junction working in the viscous unidirectional flux-flow regime. In order tomaintain fluxons moving unidirectionally under the Lorentz force, a dc magnetic field He isapplied in the plane of the junction, and the junction should have a low quality factor Qj (highquasiparticle loss ox). This means a high jc value is necessary for an FFO [6]. When the velocityof the flux-flow u approaches that of the electromagnetic wave 'E propagating along a LJJ, aresonant like current step appears in the I-V characteristic. This resulting step is commonlycalled velocity-matching step (VMS). A computer simulation illustrating flux-flow in a Lll withoverlap geometry is shown in Fig.2. The oscillation frequency can be tuned over a wide rangeby changing the applied field

f = V cic / 00 = ud.to fie/0„ (3)Here, d is the effective magnetic thickness of the junction barrier and go is the permeability offree space. The FFO was early investigated both experimentally and by computer modeling byNagatsuma et al. [2,8,9] and Zhang and Wu [10]. Relatively large output power in the submillimeter wave band was obtained for this type of oscillator, e.g., powers of about 1 p,W (intoa small junction detector) up to 400 GHz was reported early [8], and power of about 0.511,Wcoupled to a 10 C� microstripline at 280 - 330 GHz was recently reported [111. The high outputpower of an FFO is due to two reasons: i) high current density leads to a high bias current; ii)fluxons reach the output end in a tightly packed unidirectional train, thus the output voltagewaveform is nearly sinusoidal, which suggests the output power is largely concentrated at thefundamental frequency. In principle, there is no limitation to the operation frequency of this kindof oscillator up to the energy gap of the superconducting electrode. However, there were seriousdoubts about its usefulness since no linewidth measurements had been done until recently. itsoscillation frequency, which relates to the vortex density inside the junction, depends stronglyon the value of the external magnetic field. This means there exists another source of fluctuationsthat might broaden the output spectral linewidth. Ustinov et rd. [12] have measured a linewidthof 120 kHz of an FFO working at 76 GHz. Koshelets et al. [13] have used an FFO integratedwith an SIS mixer. They mixed the signal of a backward wave oscillator with the LO signalproduced by the FFO in the SIS element, and measured an upper value of the linewidth to beabout 1 MHz in the band 82 - 112 GHz.

88 Fourth International Symposium on Space Terahertz Technology

ext. _ •_.sts..:14•:•. 0.. - olgo'hif --o•

wee,: *AA:tor •-

-

."

,••.c., .31.0�•;;;-01404.

.444,fr

•-• =Nina• OA V-VY•04.-..4010*. - •

- AU*wrotrA40 A0

-„•„.

vote- --

4,0010:040:0114.--e-••• tf.o

40. tr. •IV W. .-44°A-•_,:00!$:-4-00.404-%44.

4•460sn •.4, tr, avivevirttiv-o1.% #, not 0, —to

t., •-•••••0•331%.-0-0...tt;

'felt.:

41.4.4-10.?= t_tf tt

WI -et

4.4°O. • 41.

r-

61--

,4 94i 1,14,1 fail(

-; 1sho

116r

.,5'•

26

196.6i96

r i96rf-

194.6 -time30

Fig.2 Computer simulation showing time-dependent junction voltage V(x,t) of a LB in the flux-flowregime for parameters: Laj=30, oc=0.25, f3=0.001, y=1.015 (in normalized units and at normalized magneticfield ik [101).

In this paper, we report the design of an integrated receiver in which we mix the outputoscillations from two identical fluxon oscillators in a small SIS mixer and determine thelinewidth of the oscillators from the intermediate frequency (if) with a spectrum analyzer. Thecircuit was optimized by using CAD modeling. We also show that a long-junction oscillatorcould be operated in either the resonant propagation mode or the flux-flow mode, depending onthe critical current density of the oscillator junction.

2. DESIGN

To avoid problems of coupling mm- and sub-mm waves between different wave guide systems,and to be able investigate a variety of frequencies, we decided to make a fully integrated receiverwhere the output signals (local oscillator f Lo and a weaker signal fs) of two fluxon oscillators aremixed in an SIS element to a lower intermediate frequency ( = !tip fsl ). In this way, the twofluxon oscillators and the SIS mixer could be (i) fabricated all on the same chip, (ii) coupled bymicrostripline circuitry, and (iii) analyzed using only one coaxial line and dc-bias leads.

The circuit, which was designed to operate around 350 GHz, is outlined in Fig.3. Beforedescribing the rf design of the total circuit and the CAD modeling, we will first examine therequirements on the fluxon oscillators and the SIS mixer.

2 .1 Oscillators

In Fig.4, we show a SEM photograph for a receiver circuit. The long Josephson junctions (LJJ)are constructed in an overlap geometry with a length L=4001.tm (>>X4 ). The bias current Ig isapplied perpendicular to the long dimension of the junction. In order to reduce the self field atthe end where vortices are nucleated, a projection length of 504m is employed (e.g., see left endof FFO I in Fig.4). The characteristic impedance of a LJJ of width w is given by

120n Zo = t, / d

vv-Ve r (4)


Chebychev Trans.

I I kg14 Xg14

Z 1 Z2

I Of/

4.67K2

Ro

I cont

0.560 ' I 1.07Q

FFO I

F i g . 3 The modeling circuit for measuring the linewidth of FF0s. By mixing two similar FFOs in the SISelement, we can determine the combine linewidth of the FFOs from the if product. FFOl is used as a localoscillator, while FF02 serves as the signal source.

For Nb/NbOx/PbBi junctions, the dielectric constant of the junction barrier (Nb205) Er = 29, thebarrier thickness t 2 nm, and the magnetic thickness d=t-1-. --X-Nb+XPbBi 287nm. In order tomatch the output impedance Zo of the oscillator to the load, and to avoid resonant modes alongthe small dimension of the junction, we choose a narrow width (w=-3 .trn) for the Liis(Z0=0.56 C2).

The junction critical current density, J. is an important fabrication parameter that controls the

operation mode of a LB. We have shown that a long-junction oscillator could be operated ineither the resonant propagation mode or the flux-flow mode, depending on the junction currentdensity [6]. Here, we give a rough estimate of the lowest jc value for the flux flow mode. Forsimplicity, we use the junction normal state resistance R„ instead of the dynamic resistance Rd

of the quasiparticle current at V=V dc. Thus, the quasiparticle loss a can be written as [3]

= ic (DoIcRn1j 2nC

where C is the junction capacitance per unit area. The Josephson penetration depth kj is givenby

(5)

(Do=nliojed (6)

Combining Eq.(4) and (6) together, we get

auk.) jc LwZo Rn

The condition for supporting the unidirectional flux-flow is aL /X j �.2. Taking the typical valuefor NbiNbOx/PbBi junctions lcR„= 1.8mV. we get jc � 540 A/c m 2 ( L = 4001.1m, =31.1.m,

(7)

Fourth International Symposium on Space Terahertz TechnologyPage 490

Zo = 0.56S2). Actually j, of an FFO should be at least 2 times higher than this -value, because thedynamic resistance Rd is usually a few times higher than R„.

The external magnetic field to sustain the flux motion in a LJJ can be provided by injecting dccurrent through the base electrodes or through a separate control line (another Nb layer) underthe Li-Js. Two separate local fields were applied to H-01 and FF02 (as indicated by

Iconti andIcont2 in Fig.3). For obtaining a uniform distribution of bias current along the LB, we usedmulti-finger bias leads for both base and top electrodes (see Fig.4).

2. 1 SIS detector

The SIS element should be well matched to the LJJ. A small junction area (i.e. smallcapacitance) is needed to get a reasonably large value of the tuning inductance for resonating outthe junction capacitance at 340GHz. The junction capacitance C j is inductively compensated bya strip inductance L and rf terminated by a radial stub Ro. The outer radius r, of the radial stubwas calculated approximately from [14]

r,- r 1 = X,, /2rc (8)

where the inner radius r 1 was taken as the half width of the 10 1 microstripline k g is thewavelength in the microstripline). The width of the 10 K2 microstripline is 6.91.tm for thesuperconducting case and 5.4 jim for the normal conductor. The outer radius was then optimizedby the CAD simulation and recalculated for the superconductor case ( r 2 =35.2 pm).

The LC resonance frequency, given by fo = 1/27rV LC J , is very sensitive to the length of thestrip inductance and the area of the SIS junction. Fortunately, the resonance frequency couldusually be determined from the 1-V curve of the SIS mixer, which had an additional stepstructure at the corresponding Josephson frequency (f0 = 2eV/h). From the LC resonance and bymeasuring the actual junction area in an SEM, we could get a fairly good control of the tuning ofthe SIS mixer for the second batch of samples that was fabricated. We varied junction sizes from0.5 lim 2 to 4 grn2 for different chips. The inductance length is 13.5 jim long for a junction sizeof 0.65111-n 2 . For a 4iim2 large junction, the inductor length would only be 2.2 gm long, whichshould be compared to the junction width and the microstripline width of 6.9 Jim. Hence, theactual dimensions of the inductance is very critical. To avoid this difficulty, we have also madecircuits, where two (about twice as long) inductors with separate radial stubs have beenconnected in parallel with the SIS element. The bandwidth of the SIS mixer around LC resonantfrequency fo is given by

Af 1•

Ic=fo coo R n C J .(0o(41'?, ) C (9)

Thus, the available bandwidth of the mixer is not dependent on the junction area, but depends onits Ic value [15,16]. Knowing for Nb/NbOx/PbBi junctions, IcR„= 1.8mV, C .140 fF/lim2,we acquire = 6.3 GHz for I=1 kA/cm 2 . By increasing jc ten times to 10 kA/cm 2 , we can havea quite wide bandwidth of 63 GHz. This indicates the importance of having a high j,. SISjunction for a broad band response. Since both FfOs and the SIS junction on a chip were madewithin the same process steps, they had almost the same jc value. This means that for a LJJworking in the resonant mode, the response band for an SIS detector on the same chip is verynarrow because of low j,. . That is why in experiments we found it was quite difficult to observeradiation from a low jc sample.

2.3 If circuit


Fig.4 SEM picture of the integrated receiver. The FFOs (3 j.trn wide by 350 p.rn long) are connected to an

SIS mixer via centerfed interdigital capacitors (CI and C2). T is the first section of the impedance transformer(Zi in Fi,g.3). The FFOs and the SIS element are made of NbM1b-oxide/PbBi tunnel junctions.

The rfperformances of the circuit design was optimized by careful CAD modeling (HP 85150BMicrowave Design System) with the center frequency at 350GHz. Normal conductors wereused with the resistivity set to zero. Microstripline widths and lengths were recalculated for thesuperconducting case with the London penetration depths of the electrodes taken into account[17,18]. We take XL(Nb)=85 nm and XL(PbBi)--=-200 nm [19] for superconducting microstrip-lines Nb(200nm)/Si0(400nrn)/PbBi(400nm) in the circuit. Also, we take Er = 5.5 [20] for theSiO dielectric layer with the assumption that this holds also at 350 GHz.

Both oscillator junctions are connected to the SIS mixer via center-fed interdigital capacitors (seeFig. 3). These capacitors are necessary for separating the dc-biases of the FFOs and the SISelement. One oscillator (H-401) is well coupled to the SIS mixer by using a 3-step Chebychevrnicrostripline transformer and a fairly large interdigital capacitor Cl with 20 strips (see Fig.5 fordetail). The center frequency of the Chebychev transformer [21] is 350 GHz, and its 3dBbandwidth is 80 GHz. It transforms the low impedance of the oscillator Zo . 0.5 C2) to the 10 S2microstripiine impedance through two X14 long microstriplines Z i ( = 1.07 1-2) and Z2 (.4.67 f2).The length of the finger coupler Cl has to be about k/2 long rather than 214 to give sufficientcoupling with available lithography. The problem is caused by the unfavorable small aspect ratioof the dielectric thickness (400 nm) and the strip separation (0.8 - 11.1m). CAD modeling showedthat this capacitor could have 50 GHz bandwith (3 dB loss). The second FFO has about 15 -20dB weaker coupling. It only contains four strips (63p.m long, 21.1m wide, and 2 p.m as thegap). In this way the interaction between the two oscillators is fairly small. In Fi g,.5, we showthe CAD modeled rf response of the circuit (for the normal conductor case). The transmittedpower (S12) from the local oscillator (FF01) to the SIS element has a bandwidth of 50 GHz. Anadditional window at around 100GHz, however with - 20 dB loss, is also seen in thecalculation. Although this window is very narrow, we observe radiation from the oscillator tothe SIS detector in this band.

-20-6050 190 260 400

11111111111111110


FREQUENCY [GHz

F rfcharacteristics of the modeling circuit. Microstriplines are based on lossless metals separated by a

400nm thick insulator layer with Er of 5.5. S II is the reflection from FF01, S21 is the transmitted power fromthe FF01 to the SIS element. We choose 10S2 for the resistance Rn of the SIS mixer, and 560 fF for itscapacitance Cj. Note that for the 260GHz response (see below), the actual circuit differed somewhat from this

one.

The intermediate frequency (if) signal is transmitted through a 15 C2 microstripline (41.1m wide)filtered by two pairs of radial stubs (R1 in Fig.3). The radial stubs were used as rf chokes forthe if-line. The outer radius of the radial stubs was calculated from Eq.(8), r 2 =46.1gm andr2=341.tm. The position of the if-line on the 10 SI microstrip was optimized by CAD simulation(to minimize the effect of the if-line to the rf characteristics of the circuit). The 15 SImicrostripline was then transferred to a finline via a balun construction. One electrode of thefinline (PbEii) is connected to a thin film capacitor (10pF), while the other is electricallyconnected to the Nb ground plane. The if characteristics of this circuit was simulated by CAD,and showed a flat transmission from the SIS element to IF output at the band around 4 GHz witha loss less than ldB. Before the thin film capacitor, the if-line also serves as the dc-bias line forthe SIS mixer.

3. FABRICATION PROCEDURES

Low doped silicon wafers were used as substrates. About fifty 6 x 3.8 mm 2 chips of differentdesigns were fabricated simultaneously, before dicing the wafer. The pattern of the last layer(PbBi) was defined by a direct electron beam writing for a single chip or a couple of chips at atime. This made adjustments of the circuit parameters very easy.

Both FFOs and the SIS element are fabricated by using Nb (200 nm) as base-electrodes,windows in SiO (400 nm) defining the junction areas, and PbBi (400 nm) as top-electrodes.First, a lift-off stencil for defining the Nb base electrodes is prepared with S1813 photoresist.Nb film is dc sputted at a rate of 0.5 nm/s. This Nb layer is also the ground plane for themicrostriplines. Then, on top of the Nb patterns, another lift-off stencil for defining the Aucontact pads is prepared. The Au layer is formed by thermal evaporation of 20 nm NiCr(0.1 nrn/s) followed by 250 nm Au (1 nmJs). For a good contact between the Au and Nb layers,


Ar-ion beam etching is used just before the evaporation without breaking the vacuum to providea fresh Nb surface.

Lift-off stencils for defining the junction windows in SiO as well as the top-electrodes are madeby using electron-beam writing on PMNIA/Copolymer double layer e-beam resist: copolymer(poly[methyl methacrylatemethacrylic acid] copolymer) 11% (650 nm) at the bottom andPMMA (poly[methyl methacrylate]) 3% (150 nm) on the top with an exposure dose of220 pricm2 at 50 kV. This high exposure dose works well for the lift-off process of the fingercoupler which has 20 fingers (see Fig.6). SiO was thermally evaporated at a rate of 2.5 nm/s.The Nb/NbOx/PbBi tunnel junctions were formed by Ar-ion beam etching followed by reactiveion beam oxidation and a thermally evaporated PbBi (inmis) top electrode. The critical currentdensity of the junction is controlled by the oxidation time and can be varied from 200 Alcm2 to20 kA/cm2.

F g.6 SEM micrograph of a finger coupler (CI) made of 400nm PbBi. The coupler has 20 fingers (1.4.1m

wide, 180pm long and 0. 81.trn separation). It provides good rf-coupling and dc-block between FF01 and the SIS

element. Its length is about 4/ 2 long of the center frequency for the pass band. It is the most difficult part ofthis circuit, from the fabrication point of view.

4. EXPERIMENTAL

The integrated oscillators and SIS circuit was mounted in a 22 pin IC package. Gold pads onthe chip were connected to the package by thermosonic wire bonding. The IC package wasmounted onto an IC socket at the bottom of a closed-end dipstick (0.75 inch in diameter). Twopins on the IC socket are soldered to a UT85 coaxial cable (a 1nF chip capacitor on the outerjacket of the coax gives dc-isolation also on the ground side). Bias leads were filtered by lowpass filters. The oscillators, SIS, and control currents were driven by stabilized battery sources.The chip package was shielded by Pb foil. The dipstick was immersed into all-metal shieldedhelium cryostat, and the measurements were done inside a screen room. The sample temperaturecould be adjusted from 4.2K to about 1K by pumping the liquid helium. The if signal was

Fourth International Symposium on Space Terahertz Technology

transmitted through the coaxial line to room temperature if amplifiers (fif - 4 GHz, gain 57dB)and either a square law detector or a spectrum analyzer.

For the initial circuits, the LC resonance of the SIS mixer fell outside the band of the rest of thecircuitry, and the best response of the SIS mixer was obtained at about 100 and 260GHz. Weakresponses at about 350 GHz were also seen in this case. The response was mapped out in acouple of slightly different geometries, and the passbands were well described by the predictionsobtained from CAD modeling.

4.1 Detection of resonant soliton oscillation

When jc was less than 1kAJcm 2 , strong responses were observed in the SIS detector junction toradiation at around 100GHz from resonant motion of fluxons in the LJJs. Shapiro steps andphoton-assisted-tunneling steps in the dc I-V curves of the SIS detectors agree with the radiationfrequency given by Eq.2. An independent check of the oscillation mode can be done bychanging the bias of a LE, from a resonant step (ZFS) in the positive branch to the negativebranch. For a RSO, its radiation can be detected in both bias branches, while for an FFO, thedirection of the radiation is dependent on the Lorentz force given by the vector product of thecurrent and the magnetic field. Thus, for the FFO, the velocity-matching step is morepronounced in one branch, and so is the radiation.

F i g. 7 Detection of radiation from a low j, RSO (jc =250 A/cm 2) at about 106 GHz. dc I-V curves for an SISdetector junction at 4.2 K with and without radiation. The LJJ is biased on a resonant step at V 0 4393 p,V,Iosc = 0.963 mA.

In Fig.7, we show the response of an SIS detector to the radiation from a resonant solitonoscillator that has a low jc value. When the LJJ was biased on a ZFS (-4401.1.V) enhanced by aweak magnetic field, clear photon assisted tunneling steps corresponding to the radiationfrequency (-106 GHz) appeared in the I-V curve of the detector. We believe the radiation isfrom the resonant motion of multi-fluxons in a bunched configuration. In this figure, theJosephson pair current was suppressed. When the Josephson current was present, we alsoobserved Shapiro steps in the I-V curve which corresponded to the 106 GHz radiation.


The embedding circuitry (admittance Ys.Gs+j13s as well as the magnitude of an equivalent

current source Is ) for the SIS element can be calculated from Tucker's quantum theory ofmixing [22] by using the pumped and unpumped I-V curves [23].The rf voltage amplitude Vrf

across the SIS detector can then be calculated from

7. Isv rf =

lY s Y f.k1 (10)

where Ya is the non-linear admittance at the bias point. The incident available power PLO andthe absorbed power Pde t in the SIS can be calculated from

Is-P LO =

8Gs

\Pdet V rf - r

ns-e k

riv

f,k)/ .z. (11b)

For the I-V curves in Fig.7„ we calculated Vrf= 0.77 mV, PL0 = 4.1nW, and Pde t 3 .2 nW. Forthe LJJ, the current step height times the step voltage gives 0.33 WsiV for the L.U. This dc-biaslevel fits well with the CAD modeling (i.e. about 20dB loss for this window). It also impliesthat the output power should be more than 20dB larger at the output end of the LH. For thissample, we also detected weak radiation around 106 GI-1z when the oscillator was biased on aresonant steps at around ±22OjiV. This radiation could come from the second harmonicoscillation of a resonant soliton motion which has its fundamental frequency at 53 GHz.

4.2 Detection of flux-flow oscillation

F i g.8 Detection of radiation from an FFO at about 98GHz. The dc I-V curves for an SIS detector .junction at

4.2k is shown with no radiation (a) and (b-d) for increasing LO powers. The LJJ is biased on a VM step at

V 0sc=202.6 jiV losc=1.903 mA, 1.983 mA and 2.111 mA for curves (b), (c). and (d). respectively. Negative

dynamic resistance appears at the first photon induced step for enough pumping powers. This indicates that thejunction capacitance is tuned out at this frequency. Shapiro steps were also obtained for large pump powers.

,

-1 0 1 2

I 1 I

53 4

160

Strongly pumped

Unpumped

Weakly pumped

T = 4.2K

-160

-4 -3-240

-5

1 I 1

T'240 7 I T


Measurements showed that, for LUs which had j c >1kA/cm2 , VM steps due to the unidirec-tional flux-flow motion were easily induced and their voltages could be tuned by an externalfield linearly over a wide range, typically from 0.2mV (100GHz) to 1.2 mV (600GHz), for ourNb/NbOx/PbBi junctions. As the voltage of a VM step was gradually increased with the field,the step height decreased and the step steepness decreased. This indicates that the output powerdecreases and the linewidth increases with increasing oscillator frequency. Above a certain biasclose to 1.4mV, the step suddenly disappeared, which may be due to the flux flow oscillationreaching the gap frequency.

We observed radiation from the flux-flow motion at around 100GHz and 260GHz [6], and inthe band 280 - 330GHz [11]. At these three frequencies, both Shapiro steps and photon-assistedtunneling were clearly seen in the SIS element. The oscillation mode was confirmed by changingthe bias polarity and from Eq.3. Radiations at 350GHz, 400GHz, and 600GHz were alsoobserved from the first Shapiro step at 0.72mV, 0.83 mV, and 1.24mV, respectively.

We show in Fig.8 the response of an SIS detector (Rn = 32.352) to the radiation from a flux-flow oscillator with jc=1 4kA/cm 2 . Both photon assisted tunneling steps and Shapiro stepscorresponding to radiation at 98 GHz was observed. The response became more and morepronounced (Fig.8 b-d) when the oscillator's bias was moved from the bottom of the step to thetop. The oscillator was biased at around 20311.V. Using Eqs.(10) and (11), we obtainVrf = 0.27 mV, PLO= 3.7 nW, and Pdet = 2.8 nW. Calculations also show that the SIS mixer has aconversion gain of +2dB (for curve d in Fig.8) due to the appearance of a negative dynamicresistance at the first photon-induced step. The SIS junction area was 41..1,m 2 for this sample.

VOLTAGE [mV]

F g . 9 Response of the SIS detector to radiation from FF01. For the weakly pumped I-V, the FF01 was

biased at V 0sc=651pN (315 GHz), and the calculated available power P LO is 110nW. For the strongly pumped

case, V0sc=663 iV (330GHz) and P Lo =430 nw. The same external field was supplied in both cases.


After adjusting the area of the element and the length of the tuning strip inductance carefully, theLC resonance moved to the desired center frequency, and a good response was observed from280 to 330GHz (5801tV to 69011V) with the LC resonance at 340Gliz, as shown in Fig.9.This response bandwidth (Af=--50GHz) fits well with the CAD simulation. The rf power fromthe oscillator, which could be tuned by changing the current bias along the VM step, is highenough to pump the SIS element for mixing purpose. The maximum response was at 330GHz(660pN), with a calculated power PLo= 430 WW, This sample had quite a high j c. value—12kA/cm 2 , which gave a broad detection bandwidth (around the LC resonant frequency) forthe SIS element. The SIS junction had a small junction area of 0.651= 2 , compensated by a131.1m long strip inductance.

5. LINEWIDTH MEASUREMENTS OF THE FLUX-FLOW OSCILLATOR

The linewidth was determined for the flux-flow type oscillator, i.e. for the viscous unidirectionalfluxon motion. Since two FFOs were mixed in an SIS element, the experimental set-up becamefairly elaborate. For the dc-biasing of the FFOs and the mixer we needed 5 bias supplies - 2 foreach oscillator and one for the mixer. Since the if amplifiers were operating at 4 GHz and had abandwidth of about 1 GHz, the difference in voltage bias of the two oscillators had to be in theregime 7 101.t,V. First the control currents for the oscillators were adjusted to give a velocitymatching (VM) step near a voltage corresponding to the band of interest. The response of theSIS mixer was monitored for each oscillator separately, and after adjusting the control currentsand the oscillator biases, the amplified if signal was measured with a square law detector andmonitored versus the voltage bias of the SIS element, as shown in Fig.10. This pictureresembles the typical if output power versus voltage bias of the SIS mixer. However, this maynot be a proof for mixing of two harmonic signals. Indeed, in some cases we found that eventhough the if output power from the SIS mixer was high and the bias dependence reasonable,the weakly coupled oscillator usually was too much off-set in bias from the strongly coupledone, and only an increased noise level with no additional structure could be seen when thespectrum analyzer was connected.

0 I (pumped)I (unpumped)

200

-200 1—

300 Ir

, , , , ., a , 0.8

T --.- 4.2K1\

V,,,, = 687p.V i4 \

if (pumped)- 0 - -

if (unpumped)

0.0-300

VOLTAGE [mV]

4

Fig.10 dc I-V curves of the SIS mixer and the if-output power. with and without radiation. Both FFOsoscillate at around 330GHz.

U.


FREQUENCY [5 MHz/div]

F I g. 1 1 Spectrum of the if-power at 1.4 K when the signal of FF02 is mixed with the FFOI (as the localoscillator) in the SIS mixer at around 320GHz. The center if frequency is 3.265 GHz.

After the response such as the one seen in Fig. 10 was obtained, the signal was brought to aspectrum analyzer instead. Fig.11 shows a typical recording of the if output, when spectralresponse showed mixing from coherent signals. This spectral response (with somewhatdifferent amplitudes) was seen from 280 GHz to 330 GHz. Although, the drawback of ourmethod is that we obtain the composite linewidth of the two oscillators, we do not have to dealwith an external source of a different kind, which could add uncertainties of the upper limit ofthe linewidth. The composite linewidth - 3 dB) of the FFOs in our measurement was 2.1 MHzacross this band.

6. CONCLUSIONS

An integrated receiver circuit was built to investigate radiation from long Josephson junctions.The radiation from two flux-flow oscillators was detected or mixed in an SIS mixer. ExtensiveCAD modeling of the circuit was done before the actual receiver was built. A good agreementbetween the experimental data and the modeling was obtained. We measured the linewidth of theflux-flow oscillators to be less than 2 MHz in the band 280-330 GHz. The frequency was tunedby changing the control current to the FFO. The output power within the same frequencyinterval was 0.511W coupled to a 10E2 microstripline. The output power from the oscillatorcould conveniently be tuned by changing the current bias on the velocity matching step. Byvarying the critical current density j, we could operate the long Josephson junction both in theresonant soliton regime (jc< 1kA/cm 2 ), and in the unidirectional flux-flow regime(jr > 1 kA/cm2).

ACKNOWLEDGEMENTS

We thank H. Zirath, I. Angelov and H. EkstrOm for their help, and N.F. Pederson, A.V.Ustinov and M. Cirillo for helpful discussions. The circuits were made in the SwedishNanometer Laboratory. This work was supported by the Swedish National Board for Industrialand Technical Development (NUTEK) and the Swedish Research Council for EngineeringSciences (TFR).


REFERENCES

[1] . B. Bi, S. Han, J.E. Lukens, and K. Wan, "Distributed Josephson junction arrays as local oscillators. IEEETrans. Appl. Supercond., MAG-27, to be published, (1993).

[2] T. Nagatsuma, K.Enpuku, K. Yoshida, and F. Irie, "Flux-flow-type Josephson oscillator for millimeter andsubmillimeter wave region. II. Modeling", J. Appl. Phys., 56, 3284-3293, (1984).

[3] Y.M. Zhang, "Effect of losses on the output voltage of a flux-flow type Josephson oscillator," in NonlinearSuperconductive Electronics and Josephson Devices, New York: Plenum Press G. Costabile, S. Pagano, N.F. Pederson, and M. Russo, eds., (1991), pp.145-153.

[4] N.F. Pederson, "Solitons in Josephson transmission lines," in SOLITONS, Amsterdam: Elsevier SciencePublishers B.V., S. E. Trullinger, V. E. Zakharov, and V. L. Pokrovsky, eds., (1986), pp.469-501.

[5] E. Joergensen, V.P. Koshelets, R. Monaco, J. Mygind, M.R. Samuelsen, and M. Salerno, "Thermalfluctuations in resonant motion of fluxons on a Josephson transmission line: Theory and Experiment"Phys. Rev. Lett., 49, 1093-1096, (1982).

[61 Y.M. Zhang, D. Winkler, and T. Claeson, "Detection of mrn and submm wave radiation from soliton andflux-flow modes in a long Josephson junction", IEEE Trans. Appl. Supercond., MAG-27, to be published,(1993).

[7] M. Cirillo, F. Santucci, P. Carelli, M.G. Castellano, and R. Leoni, "Coupling of long Josephson junctionoscillators at millimeter-wave frequencies", IEEE Trans. Appl. Supercond., MAG-27, to be published,(1993).

[81 T. Nagatsuma, K. Enpuku, F. Irie, and K. Yoshida, "Flux-flow-type Josephson oscillator for millimeter andsubmillimeter wave region", J. Appl. Phys., 54, 3302-3309, (1983).

[9] T. Nagatsuma, K. Enpuku, K. Sueoka, K. Yoshida, and F. Irie, "Flux-flow-type Josephson oscillator formillimeter and submillimeter wave region. III.Oscillation stability", J. Appl. Phys, 58, 441-449, (1985).

[10] Y.M. Zhang and P.H. Wu, "Numerical calculation of the height of velocity-matching step of flux-flow typeJosephson oscillator", J. Appl'. Phys, 68, 4703-4109, (1990).

[11] Y.M. Zhang, D. Winkler, and T. Claeson, "Linewidth measurements of Josephson flux-flow oscillators inthe band 280-330 GHz", Appl. Phys. Lett., to be published, (1993).

1- 121 A.V. Ustinov, T. Doderer, R.P. Huebener, J. Mygind, V.A. Oboznov, and N.F. Pedersen, "Multi-fluxoneffects in long Josephson junctions", IEEE Trans Appl. Supercond., MAG-27, to be published, (1993).

[13] V.P. Koshelets, A.V. Shchukin, S.V. Shitov, and L.V. Filippenko, "Superconducting millimeter waveoscillators and SIS mixer integrated on a chip", IEEE Trans. Appl. Supercond., MAG-27, to be published,(1993).

[14] H.A. Atwater, "Microstrip reactive circuit elements", IEEE Trans on Microwave Theory and Technology.MTT-31, 488-491, (1983).

[15] A.R. Kerr and S. Pan, "Some recent developments in the design of SIS Mixers, in First Int. Synzp. onSpace Terahertz Technology, Michigan, (1990), pp.363-377.

[16} R. Blundell and D. Winkler, "The superconductor-insulator-superconductor mixer receiver - a review," inNonlinear Superconductive Electronics and Josephson Devices, New York: Plenum Press, G. Costabile, S.Pagano, N. F. Pedersen, and M. Russo, eds., (1991), pp.55-72.

[17] W.H. Chang, "The inductance of a superconducting strip transmission line", J. Appl. Phys., 50, 8129-8134,(1979).

f181 W.H. Chang. "Measurement and calculation of Josephson junction device inductances", J. Appl. Phys.. 52,1417-1426, (1981).

1191 W.H. Henkels and C.J. Kircher, "Penetration depth measurements of type II superconducting films", IEEETrans. on Magn.. MAG-13, 63-66, (1977).

[20] H.K. Olsson, "Dielectric constant of evaporated SiO at frequency between 13 and 103GHz", IEEE Trans.Magn., MAG-25, 1115-1118, (1989).

f211 R.E. Collin. Foundations for microwave engineering, Tokyo: McGraw-Hill, 1966. Chapter 5.1221 J.R. Tucker and M.J. Feldman, "Quantum detection at millimeter wavelengths", Rev. Mod. Phys., 57,

1055-1113, (1985).1231 A. Skalare, SIS Embedding Circuit Program for Macintosh, Version 2.5. 1993.

using a cad designed integrated sub-mm wave ...by using a superconducting integrated sub-mm wave...

Documents